Various wireless communication systems such as PDC (Personal Digital Cellular), W-CDMA (Wideband Code Division Multiple Access), cdma2000 1x, PHS (Personal Handy-phone System), wireless LAN (WLAN: Wireless Local Area Network) and Bluetooth have conventionally been operated as a mobile communication system. In addition, standardization of high-speed wireless communication such as WiMAX (Worldwide Interoperability for Microwave Access) has been progressed. Each of these wireless communication systems has a different characteristic.
For example, in PHS, it is possible to take many usable terminals per unit area because a cell area is small. Therefore it has the advantage of high frequency-use efficiency. In addition, a PHS terminal is required to receive only individual call signals (PCH: Paging Channel) while connecting to a cell station (CS) and its interval is long, and thus it has also the advantage of long standby time. However, since the area covered by one cell is small, it has the disadvantage of high possibility of discontinuation of communication when communication is performed while moving at a high speed.
On the other hand, for example, W-CDMA has a wide cell area. Therefore it can be used while moving in a wide range at a high speed. However, a W-CDMA terminal performs despread processing and the like for monitoring an incoming call and a signal level of a cell and the like, and therefore its standby time is much shorter than that of the PHS terminal.
Further, with respect to WLAN as typified by widely-used IEEE (Institute of Electrical and Electronic Engineers) 802.11b, hotspots are installed in coffee shops, public facilities and the like and it can communicate at rates up to 11 Mbps. However, such WLAN is provided on the assumption of indoor use, and therefore the cell area is small such as about 10 m in radius.
Incidentally, a mobile communication terminal is generally driven by a battery. Therefore, in the case of a cellular phone terminal, for example, when a battery is consumed while in use, a power supply is turned off, and call (communication) is disabled. In this case, although functions other than call (communication) are available, the power supply of the cellular phone terminal is turned off. Therefore, functions such as an address book and the like also become unavailable, and thus it is not possible to know contact information such as a phone number when a user needs to contact someone in an emergency.
In order to solve such problem, for example, Japanese Patent Application Laid-Open Publication No. 2004-128574 proposes cellular phone terminals which allow a user to use functions such as an address book and the like to the end. The cellular phone terminals proposed gradually limit usable functions based on a remaining battery level, turn off power while keeping enough remaining battery level, or allow a user to use an address book using a sub-display that consumes less power than a main display after the power supply is turned off.